The present invention relates generally to apparatus for protecting electrical equipment from damage or destruction due to the presence of electrical overvoltages, such apparatus commonly referred to as a surge arrester. More particularly, the invention relates to a non-fragmenting, surge arrester. Still more particularly, the invention relates to an elastomer housed distribution arrester having a staged pressure relief system which, in the unlikely event of failure, safely vents ionized gases generated by internal arcing outside the arrester, thereby preventing what otherwise could be a catastrophic failure of the arrester.
A surge arrester is commonly connected in parallel with a comparatively expensive piece of electrical equipment in order to shunt overvoltage surges, such as those caused by lightning strikes, to ground, thereby protecting the equipment and circuit from damage or destruction. A modern surge arrester typically includes an elongated enclosure made of an electrically insulating material, a series of voltage dependent nonlinear resistive elements retained within the housing, and a pair of electrical terminals at opposite ends of the housing for connecting the arrester between line and ground. The voltage dependent nonlinear resistive elements employed are typically, but not restricted to, metal oxide varistor elements formed into relatively short cylindrical disks which are stacked one atop the other within the enclosure. Other shapes and configurations may also be used for the varistor elements. The varistor elements provide either a high or a low impedance current path between the arrester terminals depending on the voltage appearing across the varistor elements themselves. More specifically, at the power system's steady state or normal operating voltage, the varistor elements have a relatively high impedance. As the applied voltage is increased, gradually or abruptly, the varistor elements' impedance progressively decreases until the voltage appearing across the varistors reaches the elements' breakdown voltage, at which point their impedance dramatically decreases and the varistor elements become highly conductive. Accordingly, if the arrester is subjected to an abnormally high transient overvoltage, such as resulting from a lightning strike or power frequency overvoltage for example, the varistor elements become highly conductive and serve to conduct the resulting transient current to ground. As the transient overvoltage and resultant current dissipate, the varistor elements' impedance once again increases, restoring the arrester and electrical system to their normal, steady-state condition.
Occasionally, the transient condition may cause some degree of damage to one or more of the varistor elements. Damage of sufficient severity can result in arcing within the arrester enclosure, leading to extreme heat generation and gas evolution as the internal components in contact with the arc are vaporized. This gas evolution causes the pressure within the arrester to increase rapidly until it is relieved by either a pressure relief means or by the rupture of the arrester enclosure. The failure mode of arresters under such conditions may include the expulsion of components or component fragments in all directions. Such failures pose potential risks to personnel and equipment in the vicinity. Equipment may be especially at risk when the arrester is housed within the equipment it is meant to protect, as in the tank of a transformer for example.
Attempts have been made to design and construct arresters which will not catastrophically fail with the expulsion of components or component fragments. One such arrester is described in U.S. Pat. No. 4,404,614 which discloses an arrester having a non-fragmenting liner and outer housing, and a pressure relief diaphragm located at its lower end. A shatterproof arrester housing is also disclosed in U.S. Pat. No. 4,656,555. Arresters having pressure relief means formed in their ends for venting ionized gas in a longitudinal direction are described in U.S. Pat. Nos. 3,727,108, 4,001,651 and 4,240,124.
Despite such advances, however, state of the art arresters may still fail with expulsion of components or fragments of components. This may in part be due to the fact that once the internal components in these arresters fail, the resulting arc vaporizes the components and generates gas at a rate that can not be vented quickly enough to prevent rupture of the arrester. Accordingly, there exists a need in the art for an arrester which, upon failure, will fail in a non-fragmenting manner. Preferably, such an arrester would eliminate the possibility of catastrophic failures by transferring the failure-causing arc away from the internal components, thereby preventing the generation of any additional pressure. One means by which this may be accomplished is to design an improved arrester which would safely vent the ionized gases formed by the internal arc outside the arrester, thereby forming a lower impedance path to ground for the arc. It is further desirable that such an improved arrester would vent the gases in a staged or controlled manner so as to prevent fracturing the arrester by an abrupt change in internal arrester pressure.